Introduction to Defoamers – Part 2

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In last article, we have discussed about some basic concepts and mechanism of defoamers. There are many materials could be used to make defoamers but only a small percentage of them will have excellent properties. Today, we will discuss more on silicone oil defoamers more and mainly on the mechanisms of inactivation & regeneration of silicone oil defoamers and how they will influence the effects of properties of silicone oil defoaming agents.

Inactivation & Regeneration of Silicone Oil Defoamers

The inactivation & regeneration events of defoamers are found by the defoaming experiment made by Krastanka & Denkov by adding silicon ointment defoamers and emulsion silicon defoamers into the AOT solvent. To be more specific, to add 0.01% silicon ointment or emulsion defoamers to 100ml and 250ml 10mmol/L AOT solvents respectively and do the AST. The result of the experiment shows it is in the first 15 to 20 times of shaking, the defoamer displays a high activity of defoaming, then the time of defoaming increases rapidly. When the number of times of shaking is over 45, the time of defoaming lasts at 60 seconds and when over 60 seconds the defoamer becomes inactive. By adding 5μL silicone oil to the inactive AOT solvent, the activity of defoaming recovers. When we do another AST, the defoamer turns to be inactive again. By adding another 5μL silicone oil, we could get defoaming solvent again. It is according to the results above, we could find that the inactivation and regeneration phenomenon are presented one by one in a recycle.

The Mechanism of Inactivity of Silicone Oil Defoamers

There are few opinions discussing to try to explain the inactivity of silicone oil defoamers.

1.      The changes on the components of defoamer. The silicone oil defoamer usually consists of some different types of silicone oil, and some of them are easier to leave oil phases and soluble into the micelle of the surface active agent micelles which leads to the changes of the components of defoamer oil phases then lower the activity of defoamers.

2.      The size of particles of dispersion of silicone oil defoamer. Kozo thinks the main reason why defoamer is inactive is because the particle size of dispersion of silicone oil defoamers. He observes that the particle size of dispersion of new defoamers will be in 2 ~ 30μm during defoaming process into 1~3μm, Berg-eron also observes the particle size of defoamers will be from 30μm to 8μm. The small particle size will lead to a stronger effect between dispersion of defoamer and foam film, then lead to a longer time of water releasing from foam films and finally make a longer time of defoaming.

3.      The disappearance of spreading layers of silicone oil defoamers. The silicone oil player which is produced on the surface of foam films is the essential requirement for defoaming. However, it is during the defoaming process, the silicone oil player will be emulsificated gradually into the solvent and make the defoaming ability to be lower.

4.      The separation between silicone oil and silicon particles. This opinion is mentioned firstly by Kulkarni, he thinks the hydrophobic solid particles of silicone oil defoamer system which is applied as the active components are contacted with surface active agents for a long time, they will be gradually leave from the dispersion of defoamers to the solvent. These dispersions which have lost silicon particles will fail to enter the surface of foamer films due to high insertion resistance. Also, some silicon molecules which have entered the foam films will be attached to the solid particles then enter the solvent to make the hydrophobic becomes lower and the hydrophilicity becomes stronger and finally leads to the loss of defoaming function.

Pouchelon and Araud have observed the inactivity of defoamers is acting with the production of some white flocculants.

The Mechanism of Regeneration of Silicone Oil Defoamers

Denkov has observed the white flocculants will disappear when adding silicone oil, he thinks the new-added silicone oil has diluted the flocculants. Although the new-added silicone oil has a low inactivity, but when entering the foaming system, the silicone oil will be redistributed in the inactive defoamer dispersion and flocculants and to reform new dispersions with high defoaming activity.

For emulsion defoamers, the particle size of newly formed defoamer emulsion dispersions will gave big influence on the regeneration of defoamers. When the particle size is larger (over 90μm averagely), then adding silicone oil, the spreading oil player which is formed by most of newly formed defoamer dispersion entering the surfaces of foams will be equal to the spreading oil layer which are formed when defoamers enter solvents. The silicone oil layer will increase during the processing of defoaming due to it will have emulsification to become inactive. Therefore, the activity of defoaming is strong and the effect could last longer. When the particle size is smaller (average 2μm), only some parts of the newly formed defoamer dispersion liquid drips will be inserted into the surfaces of foams and to form unstable oil bridge to make the film broke. It means that the quantity of dispersions with defoaming activity is small and the oil layers which are spread over the surface of foams will be emulsificated rapidly to become inactive, thus the defoamers will gave worse defaoming activity and shorter lasting period after being regenerated.  

Summary

To compare with other silicon products, it is true that the mechanisms of silicone oil defoamers have more complex mechanisms and even some of them are in debates. However, these will not influence the high commercial and industrial values of silicone oil defoamers especially they have already performed successfully in different application industries.

In next article, we will introduce more on different types of silicone oil defoamers and the unique properties owned by different type of it.